In recent years, the development of the technique of Spatial Codec has been encouraged. This technique aims at compressing and coding multiple-channels with a very small amount of information while maintaining a sense of realism of sounds from the multi-channels.
For example, a bit rate of 512 kbps or 384 kbps is required for 5.1 channel sound by the AAC format which is a multi-channel codec that has been widely used as an audio method used for digital televisions. In contrast, Spatial Codec aims at compressing and encoding multiple-channel signals at a very small bit rate such as 128 kbps, 64 kbps, or further still, at 48 kbps. For example, Patent Reference 1 describes a technology for realizing the above.
According to Patent Reference 1, the sense of realism is maintained by compressing and encoding a ratio of levels between channels (hereinafter referred to as “level difference” and “gain ratio”).
More specifically, in binaural cue coding (BCC), spectral components of an input signal are downmixed so as to generate a BCC parameter (for example, an inter-channel level and/or time difference). When implementing the generated BCC parameter in stereo, bilateral-channel signals are transformed into signals in a frequency domain and the spectral components of a pair of the bilateral-channel signals are then downmixed into mono components. After that, these mono components and spectral components of the bilateral-channel signals which have not been downmixed are inversely transformed into signals in a temporal domain so as to generate a hybrid stereo signal, and the hybrid stereo signal is encoded using conventional coding techniques. The encoded bit stream is decoded by conventional decoding techniques and reproduced. Next, an auditory scene is synthesized based on the mono components and stereo components that have not been downmixed, by applying the BCC parameter using the BCC synchronization method.
Therefore, when actually viewing and listening using a home AV device and so on, an encoded downmixed signal is divided using auxiliary information so that signals can be divided to an extent in which there is still no auditory sense of discomfort. In addition, in the case of easy listening through headphones of a cellular phone and the like, only a downmixed signal alone needs to be decoded without using a BCC parameter so that signals can be reproduced in good sound quality with easy and fewer calculations, which cannot be realized by the conventional compression methods.
Patent Reference 1: United States Patent No. 2003/0236583A1 (which corresponds to Japanese Laid-Open Patent Application No. 2004-78183)